Does Domestication Produce Dummies?

ResearchBlogging.orgDomestication is by far man's greatest genetic experiment, and we've been at it for well over 10,000 years. While domestication can produce wild variation (see my post on dogs, for example), a few changes seem to be universal. These include behavioral changes, like reduced fear of humans and friendliness, as well as physiological ones, like floppy ears (they develop in domesticated foxes, too). One of the most well-documented differences between domesticated animals and their wild counterparts is their brains: across every species that has been looked at, the brains of domesticated animals are smaller. Why does domestication lead to shrinking brains? And what does this mean in terms of intelligence?

Allometry is the study of the relationship between size and shape of things, and how these relate to their functions. Allometric methods are often applied to understanding brains and intelligence and how they may differ between animals, especially when looking at non-living creatures that can't be put through behavioral tests. Of course, you can't just look at the size of a brain all by itself. After all, by weight, elephant brains are far larger than ours, and sperm whale brains are even larger than theirs! But that doesn't tell us anything about intelligence, since brains are bound to be bigger in larger animals in general. Instead, scientists look at the ratio of brain size to body size, not just the weight or width of the brain itself. The amount of brain mass exceeding what would be expected by an animal's total body mass is termed it's degree of 'Encephalization', or its 'Encephalization Index', which is thought to relate directly to intelligence. We have a higher degree of encephalization than apes, for example, and they in turn have a higher degree than most other animals.

brain decrease in domestication.pngThe table on the right (Kruska, 2005) shows the Encephalization Indicies (EI) and the percent decrease in relative brain size (DV, %) between wild and domestic forms of different species. A wide variety of domesticated animals, from horses to rats, all show decreases in relative brain size (FYI this is true for domesticated birds, like geese and ducks, too). The trend is stronger in species that had higher degrees of encephalization begin with, like pigs. Since encephalization is supposed to correlate directly with intelligence, this would indicate that our domesticated animals are much dumber than their wild relatives, particularly those that were smart to begin with. Furthermore, studies have supported that these differences are indeed genetic rather than environmental in nature, as crosses between wild and domesticated versions have brains in the middle of the two. Also, captive-bred wild animals do not show this marked decrease in brain size, so it's not likely the captivity itself alters the size of animal brains, and domestic animals that become feral don't show an increase in brain size over time.

Why might domestication have bred stupidity? Well, there is the obvious reason: intellect isn't a trait we selected for first. Horse breeders were likely more consumed with whether their studs ran fast than if they could learn to push a certain shaped button for food. Similarly, people wanted cute guinea pigs that were easy to hold, not ones that could memorize a maze. Indeed, there are potential downsides to a smart pet: smarter animals might be more able to get out of enclosures or be otherwise devious, making them harder to keep. Furthermore, it's possible that intelligence is positively correlated with a trait we really don't want, like aggression, so that by breeding for other desirable traits, we lowered intellect accidentally. It's also possible that we started out with dumber stock - after all, the smartest geese might have been those we couldn't catch.

But is this really true? Are dogs and guinea pigs stupider than wolves and cavies?

In many cases, the answer appears to be yes. Dogs, for example, appear to be a few crayons short of the box when compared to wolves. A study in 1985 found that wolves vastly outperformed malamutes in getting a food dish from a series of complex puzzle boxes. And while one study had found that dogs are better able to pick up human social cues than wolves, they were criticized for using wild wolves that had little interaction with people. Indeed, when another set of researchers did the same kind of study using wolves that had been raised by humans, the wolves beat the dogs hands down.

But not everyone buys into the smaller-brains-dumber-animals hypothesis. In their review of a number of brain size studies, Healy & Rowe concluded that whole brain size, by itself, was not a useful variable "when considering the role of selection in the relationship between brain and behaviour." They cite numerous studies of primates and birds which fail to find a relationship between whole brain size and indicators of intelligence like social complexity.

They're backed up by studies which have found domesticated animals, despite their smaller brains, fare just as well as wild ones. Rodents, in general, don't seem to have suffered from domestication. Wild rodents don't outperform domesticated ones on a number of tests, and often, it's the lab rats that do better than their undomesticated cousins. For example, experiments using a number of different rat strains found that inbred and albino rats showed decreased learning abilities compared to wild ones, but the non-inbred domestic strains did just as well. Furthermore, a study just published online first for Frontiers in Zoology found that domestic guinea pigs found an underwater platform much faster than wild cavies. They conclude that in the case of guinea pigs, "artificial selection for human desired traits did not led to a degeneration of cognitive capabilities."

Of course, all of this is confused by the fact that "intelligence" is an amorphous and often hard to quantify concept, particularly in anything that isn't human. While we can give people SATs or IQ tests (which are controversial enough as measures of intellect), pinning down whether an animal is 'smart' is much, much harder. Do we define intelligence as social complexity and their interactive memory of their peers? Ability to learn behaviors when trained? Problem solving? Self-recognition? The battery of tests for 'intelligence' in animals is almost endless, and few of them have been given to wild and domesticated species in a comparable setting.

In the end, it's hard to say if domesticated animals are in truth dumber than their wild relatives without further, more rigorous studies. If they are, though, would it really be that shocking? We don't prize intelligence all that highly in our own society - just look at the popularity of celebrities like Paris Hilton. Why should our animals be any different?

Kruska, D. (2005). On the Evolutionary Significance of Encephalization in Some Eutherian Mammals: Effects of Adaptive Radiation, Domestication, and Feralization Brain, Behavior and Evolution, 65 (2), 73-108 DOI: 10.1159/000082979

Frank, H., & Frank, M. (1985). Comparative manipulation-test performance in ten-week-old wolves (Canis lupus) and Alaskan malamutes (Canis familiaris): A Piagetian interpretation. Journal of Comparative Psychology, 99 (3), 266-274 DOI: 10.1037/0735-7036.99.3.266

UDELL, M., DOREY, N., & WYNNE, C. (2008). Wolves outperform dogs in following human social cues Animal Behaviour, 76 (6), 1767-1773 DOI: 10.1016/j.anbehav.2008.07.028

Healy, S., & Rowe, C. (2007). A critique of comparative studies of brain size Proceedings of the Royal Society B: Biological Sciences, 274 (1609), 453-464 DOI: 10.1098/rspb.2006.3748

Harker KT, & Whishaw IQ (2002). Place and matching-to-place spatial learning affected by rat inbreeding (Dark-Agouti, Fischer 344) and albinism (Wistar, Sprague-Dawley) but not domestication (wild rat vs. Long-Evans, Fischer-Norway). Behavioural brain research, 134 (1-2), 467-77 PMID: 12191833

Lewejohann L, Pickel T, Sachser N, & Kaiser S (2010). Wild genius - domestic fool? Spatial learning abilities of wild and domestic guinea pigs. Frontiers in zoology, 7 (1) PMID: 20334697


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Well the majority of food animals on farms are so dumb they die with out daily intervention of human caretakers. Breed for docile to make handling easier and get dumb as rocks with it.

I think it would be especially troublesome to test intelligence using food as a reward, considering the huge difference between hunting and feeding for survival over thousands of years. I have no problem believing that domestication makes animals dumber since it's not the selected trait, it just seems to be an especially difficult problem to test. Maybe we need to get some wild humans to figure it out.

My wife and I have often wondered if human have domesticated themselves in this fashion. After all, don't humans who get along well in human society have a reproductive advantage? We likely have improved our impulse control (don't know how we'd test that).

Do we have smaller brains than early modern humans? Or are we more neotenous? I suspect much of our intelligence came from understanding and manipulating and lying to our fellow humans. I wonder if we have gotten better at that from living among so many of us, or simply more compliant?

I think comparing rats and dogs is a little off. They are domesticated for different purposes.

Generally I would think docility would be associated with lowered intelligence. Being intelligent means being able to think, "Stuff that. *You* go fetch the red squeaky ball."

I'm surprised by the results you mention of wolves reliably picking up human social cues. I thought the original study actually was done on captive bred "tame" wolves who grew up with routing human interactions. A summary or link would be appreciated.

As for "dogs" generally... I expect to see marked differences between working dogs and pet dogs. This is a bit difficult these days, since the Victorians did a pretty thorough job inbreeding the hell out of all the different breeds to codify arbitrary traits (even the supposedly working breeds). Still, a good hunting or herding dog has (mostly at least) been selected for some degree of a specific suite of cognitive abilities (we would typically call intelligence).

Overall, my intuition is that everything depends on what specific sort of cognitive abilities are being measured. It is to be expected that the sort of intelligence displayed by non-human species will be quite alien. Primates may be somewhat similar, but rodents and even canines are not very much like us. (Dogs are a special case since they have co-evolved in cooperation with us for a while.) By the time we get to birds and cephalopods all we can really do is look at how effective they are (or notions of different specific mental abilities are just not applicable IMO).

Are there any objective studies of intelligence difference between dog breeds? Intuitively, border collies are more intelligent than, say, pekingese. But that intuition might be observer bias combined with differential cultural uses of the dogs. But to the extent that dogs have been bred for a range of purposes from the ornamental to the functional, it seems important to differentiate breeds -- or at least consider that range -- when investigating the effects of domestication.

Horse breeders were likely more consumed with whether their studs ran fast than if they could learn to push a certain shaped button for food.

Not all horses are selected for speed, and some breeds are known for their intelligence and trainability (ever watch a dressage routine? That takes some serious skill on both the human and equine parts). Only a small portion of the horse breeds that exist are used exclusively for racing (big names: Arabians, Quarter horses, and throughbreds).

(Sry for posting again..)

@Kermit: I recall from one of my classes a few years go that Neaderthals had a higher encephalization index then us. I don't have a refrence handy though.

Do all dog breeds have the same encephalization? After all, there is a huge range of dog sizes, and I am not certain that dog heads scale exactly the same. Imagine a chihuahua proportioned head on a great dane.

That was a lot of of work just to make a crack on Paris Hilton.

I think it may be the other way round (ie no longer selecting for intelligence, rather than selecting for stupidity). Large brains have a very high metabolic cost, and if an animal is getting food given to it and is protected from predators, a large brain will offer less of a survival advantage.

By stripey_cat (not verified) on 02 Apr 2010 #permalink

Does encephalization work in the opposite direction? Do wild horses have bigger brains? Some of them have been wild [in N America] for more than 400 years.

By natural cynic (not verified) on 03 Apr 2010 #permalink

I think this question would unfold itself if we came up with a cognitive definition of what it means to be tame. Interesting theories about natural skittishness and boldness in animal population - there must be genetic diversity in this area so that the species can affect generational adaption to temporal changes in predator threats. Where the domestic dogs the one that had a naturally less intense flight instinct and were able to live in closer company to an extremely dangerous predator - man?
Has anyone done any experiments on breeding super-intelligent dogs? I have a friend who breeds sheep dogs, favoring the ones that can respond most accurately to complicated verbal commands. He reports that after a dozen or so generations his dogs are highly intelligent, but that many of them tend to have a naturally nervous disposition. Not aggressive, just a little crazy.

By Falafel Getmemore (not verified) on 03 Apr 2010 #permalink

I wonder if neoteny (retention of juvenile traits in adult forms) could partly explain this. Domesticated dogs retain many youthful behaviors through their lives -- interest in play, barking, etc. Domesticated animals may also have more youthful "personality," in terms of muted aggressiveness. As most animals become more intelligent as they age, if cognitive aging is unintentionally suppressed, the result could be a lessened "intelligence." Hmm...

It's not fair to generalize from general domestication patterns to dogs specifically, because the domestication of dogs is very different from the domestication of feed animals. Yes, domesticating cows, chickens, and turkeys no doubt made them stupider. But "domestication" in those cases meant making them more sedentary, and breeding them for qualities which don't include intelligence. Dogs, on the other hand, have different breeding pressuresâthey interact so regularly and so tightly with humans that a certain kind of intelligence is extremely valuable. Indeed, in many cases, a certain kind of intelligence difference between life and deathâcertainly the difference between breeding and not. This isn't generally true of domesticated creatures.

Further, given that dogs are better at reading human social queues and performing social tasks than chimps and wolves (… and…), I'm immediately suspicious of any declaration about what it means for domestication to produce intelligence. Dogs, for instance, are much better at vocal queues than wolves: wolves, on the other hand, are better at reading visual queues.

Additionally, I'm suspicious about encephalizationâlike Mark P, I note a proportional difference in brain size which is not correlated with smarter breeds. Jack Russels, for instance, are brilliant dogs, but chihuahuas are neurotic morons with proportionally larger heads. I need more evidence before I'll buy encephalization as an argument between two creatures within effectively the same species (i.e. dogs and wolves).

Like human IQ tests, I suspect that it all boils down to what metrics we use for "intelligence". Domesticated dogs may have given up the ability to figure out puzzle boxes, but they figured out how to work better in society. Is this a net loss in intelligence?

I just noticed the Udell, Dorey, and Wynne article. Interestingâso it looks like given enough human interaction, dogs and wolves can both figure out the human social queue of pointing. This puts them both one up on chimps.

Looking at the discussion in the paper, their claim that wolves outperform dogs is a bit of an overstatementâbut they're comparable as long as the wolves are exposed to humans early and often enough. The fact that they tested dog cognition in a shelter puts their comparison numbers deeply at doubt, which is somewhat ironic since their whole study is a critique of the environmental set-up of the Hare et al. study.

As I composed my reply to a NewScientist article that strangely never showed up on that page, my research lead me to this post.
The fact that domestication reduces brain size, that human brain size correlates to IQ (#14 @Bryan), and that human brain size has shrinked over the last 30,000 years or so supports my notion that we humans are domesticated by our societies, which in turn is supporting evidence in favor of Fauceir Theory.